Interactive effects of manganese addition and simulated ocean warming differ between hard and soft corals

Marine heat waves pose a critical threat to hard corals and mitigation strategies are needed to support coral survival. Manganese (Mn) is an essential micronutrient whose addition can alleviate thermal stress in some hard corals. However, the effective dose as well as potential detrimental effects at higher dosages are largely unknown and may be species-specific. To assess the suitability of Mn supplementation as an intervention measure, it is further crucial to test its effect on other coral reef organisms, such as soft corals. We investigated the combined effect of Mn supplementation (daily pulses of 0, 120 and 260 µg L-1) during a simulated heatwave (stepwise increase from 26 to 32 °C) on the physiology of the hard coral Montipora digitata and the pulsating soft coral Xenia umbellata in a 5-week aquarium experiment. Mn addition did not have a significant effect on both corals at the culture temperature of 26 °C. M. digitata was increasingly affected by rising temperatures with severe bleaching and a 90% reduction in symbiont density at 32 °C, independent of Mn concentration. In contrast, X. umbellata was unaffected by increasing temperature alone, but in combination with Mn enrichment, polyp pulsation was reduced by 20% (120 µg L-1) and 44% (260 µg L-1) over the last week at 32 °C, respectively. Corroborative semi-quantitative MRI measurements further indicated a higher Mn incorporation by X. umbellata compared to M. digitata, suggesting species-specific differences in trace metal accumulation. These findings emphasize the importance of species-specific differences in hard corals and other coral reef organisms, when assessing the potential of Mn enrichment as an intervention measure to increase heat resistance in hard corals.